The present invention relates to a rolling-bearing unit for a wheel which is called a fourth-generation hub unit and is used for supporting the driven wheels {front wheels for a FF vehicle (front-engine front-wheel drive vehicle), rear wheels for a FR vehicle (front-engine rear-wheel drive vehicle) and RR vehicle (rear-engine rear-wheel drive vehicle) and all wheels for a 4WD vehicle (four-wheel drive vehicle)} that are supported by an independent-type suspension such that they rotate rotate freely with respect to the suspension device.
In order to support the wheels such that they are able to freely with respect to the suspension device, various kinds of rolling-bearing units for wheels have been used which comprises an outer race and inner race that rotate freely by way of rolling members. Moreover, a rolling-bearing unit for wheels that is used in an independent-type suspension device for supporting the driven wheels, must be combined with a constant velocity joint and smoothly (maintaining constant velocity) transmit the rotation of the drive shaft to the wheels regardless of the relative displacement of the differential gear and drive shaft or of the steering angle applied to the wheels. This kind of rolling-bearing unit for wheels, which is combined with a constant velocity joint, and can be configured such that it is relatively compact and light weight, is called a fourth-generation hub unit, and has previously been known as disclosed in Patent Publication No. Toku Kai Hei 7-317754.
FIG. 1 shows the construction as disclosed in the aforementioned patent publication. With rolling-bearing unit installed in the vehicle, an outer race 1 (outer ring like member) is supported by the suspension device such that it does not rotate, and it has a first installation flange 2 formed around its outer peripheral surface for attaching it to the suspension device, and multiple rows of outer-ring raceways 3 formed around its inner peripheral surface. On the inside of this outer race 1, there is a hub 6 which comprises a first inner-race member 4 and a second inner-race member 5. Of these, the first inner-race member 4 is of a cylindrical shape and has a second installation flange 7 for supporting the wheel formed on one end side (left end side in FIG. 1), and a first inner-ring raceway 8 formed around its other end side (right end side in FIG. 1) around its outer peripheral surface, respectively. With respect to this, the second inner-race member 5 has a cylindrical section 9 on one end (left end in FIG. 1) for fitting around and attaching to the first inner-race member 4, and a housing portion 11, on its other end (right end in FIG. 1), which acts as the outer ring of a constant velocity joint 10 of the Rzeppa-type and a second inner-ring raceway 12 formed around its outer surface in the center section. Moreover, by placing multiple rolling member 13 between the outer-ring raceways 3 and the first and second inner-race raceways 8, 12, it is possible to support the hub 6 on the inside of the outer race 1 such that it can rotate freely.
Also, attachment grooves 14, 15 are formed around the inner peripheral surface of the first inner-race member 4 and the outer peripheral surface of the second inner-race member 5 in the location where they come together, and a stopping ring 16 is placed in both of these attachment grooves 14, 15 to prevent the first inner-race member 4 from coming out of the second inner-race member 5. Furthermore, welding 18 is performed between the outer peripheral edge of one end face (left end face in FIG. 1) of the second inner-race member 5 and the inner peripheral edge of a step section 17 that is formed around the inner peripheral surface of the first inner-race member 4, and it joins and fastens the first and second inner-race member 4, 5 together.
Also, between the opening on both ends of the outer race 1 and the outer surface in the center of the hub 6, there are cylindrical shaped covers 19a, 19b that are made of metal such as stainless steel, and ring shaped seal rings 20a, 20b that are made of an elastic material such as rubber or elastomer. These covers 19a, 19b and seal rings 20a, 20b isolate the area where the multiple rolling members 13 are located from the outside, and they present the grease in that section from leaking out, as well as foreign matter such as rain water or dirt from getting inside this section. Moreover, on the inside of the center section of the second inner-race member 5, there is a partition plate 21 that closes off the inside of this second inner-race member 5, and it helps maintain the rigidity of the second inner-race member 5, as well as prevents foreign matter that gets inside this second inner-race 5 through the opening on the tip end (left end in FIG. 1) of this second inner-race member 5 from reaching the section of the constant velocity joint 10 that is located inside the housing 11.
Moreover, the constant velocity joint 10 comprises the housing 11, inner ring 22, retainer 23 and multiple ball 24. Of these, the inner ring 22 is driven and rotated by the engine by way of the transmission, and is attached to the tip end of the drive shaft (not shown in the drawings). Around the outer peripheral surface of this inner ring 22 there are six inside engagement grooves 25 which have a circular arc shaped cross section when cut along an imaginary plane that crosses the center axis of this inner ring 22, and they are spaced at equal intervals around in the circumferential direction and are orthogonal to the circumferential direction. Also, around the inner peripheral surface of the housing 11 at locations that face the inside engagement grooves 25, there are six outside engagement grooves 26 that also have a circular arc shaped cross section and arc orthogonal to the circumferential direction. Moreover, the retainer 23 is ring shaped with a circular arc shaped cross section and it is held in place between the outer peripheral surface of the inner ring 22 and the inner peripheral surface of the housing 11. At six locations in the circumferential direction around this retainer 23, at positions that is in alignment with the inside and outside engagement grooves 25, 26, there are pockets 27, and a ball 24 is held in each of the six pockets 27 for a total of six balls 24. These balls 24 are held in the pockets 27, and are able to rotate freely along the inside and outside engagement grooves 25, 26.
When the rolling-bearing unit for wheels constructed as described above is installed in a vehicle, the outer race 1 is supported by the suspension device by the first installation flange 2, and the driven wheel is attached to the first inner-race member 4 by the second installation flange 7. Moreover, the tip end of the drive shaft (not shown in the drawings) is rotated and driven by the engine by way of the transmission, and it is connected to the inside of the inner ring 22 of the constant velocity joint by splines. When the vehicle is operated, the rotation of the inner ring 22 is transmitted to the hub 6, which includes the second inner-race member 5, by way of the multiple balls 24, and rotates and drives the driven wheel.
In the case of the prior rolling-bearing unit for wheels shown in FIG. 1, it is difficult to make it compact and lightweight. The reason for this is as follows. When transmitting rotating power between the inner ring 22 and the housing 11 of the constant velocity joint 10, the inside and outside engagement grooves 25, 26 and the ball 24 displace relative to the direction of contact between the rolling contact surface of the ball 24 and one side surface in the circumferential of the engagement grooves 25, 26, as shown exaggeratedly in FIGS. 2 and 3. As a result of this relative displacement, the balls 24 are displaces such that they ride up toward the opening of the inside and outside engagement grooves 25, 26. At the same time, the rolling contact surface of the ball 24 and the inner surface of the inside and outside engagement grooves 25, 26 come in contact with each other in the area of a contact ellipse whose long diameter is in the circumferential direction of the constant velocity joint 10. On the other hand, the inside and outside engagement grooves 25, 26 have chamfered surfaces 28a, 28b that are formed along the entire length of the engagement grooves 25, 26. The contact ellipse of the ball 24, due to the aforementioned displacement, moves toward the opening of the engagement grooves 25, 26, and when the contact ellipse reaches the chamfered surfaces 28a, 28b, edge load Pe, as shown in FIG. 3, occurs at the end edge of the chamfered surfaces 28a, 28b. This kind of edge load Pe reduces the rolling fatigue life of the rolling contact surface of the ball 24, as well as reduces the durability of the constant velocity joint 10, which is undesirable.
Therefore, up until now, in order to maintain the necessary durability, regardless of the reduction in rolling life due to the causes above, the outer diameter of the ball 24 was increased so that there is a margin in the load capacity on the balls. By increasing the outer diameter of these balls 24, the constant velocity joint 10 became larger, and the overall weight of the rolling-bearing unit for wheels was increased by that amount. Increased weight of the rolling-bear unit for wheels increases the unspring weight of the vehicle, so that it is desired to make this rolling-bearing unit for wheel more compact and light weight.